In analysis of road traffic conditions, the traveling status of each vehicle, for example, whether the vehicle has stopped sometimes becomes an important element. Recently, determining whether the vehicle has stopped, using a Global Positioning System (GPS) has been put into practical use.
The GPS is a positioning system for locating (i.e., positioning), on the basis of a signal transmitted from a plurality of artificial satellites above the sky (hereinafter, referred to as “GPS signal”), the current position of a receiver that receives the GPS signal. Installing the receiver in a vehicle to calculate the time difference in positions from time series data of current positions, for example, enables measurement of the velocity of the vehicle at the respective times.
However, due to the principle of GPS, the lower the moving velocity of the receiver becomes, the lower the detection accuracy of time difference in positions becomes. Accordingly, when the moving velocity of the receiver is low (e.g., about 10 km/h or lower), the measurement simply using the time difference in positions cannot measure velocities with high accuracy, thus resulting in only low accuracy measurement. That is, it is impossible to determine accurately whether a vehicle has stopped on the basis of the velocity measurement simply using the time difference in positions.
Under such circumstances, Patent Literature (hereinafter, referred to as “PTL”) 1 and PTL 2, for example, disclose a technique of determining that a vehicle has stopped, on condition that the velocity measured from time differences in positions is equal to or lower than a threshold value higher than zero. According to the technique disclosed in PTLs 1 and 2, setting the threshold value sufficiently high makes it possible to determine more securely that a vehicle has stopped when there is a possibility that a vehicle has stopped.
However, the technique according to PTLs 1 and 2 may determine that a vehicle has stopped although the vehicle has not stopped in reality. Meanwhile, setting the threshold value lower in order to prevent such a false determination reduces the accuracy of the determination whether or not the velocity is equal to or lower than the threshold value. In this case, the possibility of determining that the vehicle has stopped although the vehicle has not stopped in reality may rather increase. Further, the possibility of determining that a vehicle has not stopped although the vehicle has stopped in reality may also increase.
Under such circumstances, PTL 3, for example, discloses a technique of using yet another condition to determine whether a vehicle has stopped. The technique disclosed in PTL 3 measures the direction of a velocity vector from the time difference in positions. The technique disclosed in PTL 3 determines that a vehicle has stopped on conditions that a velocity measured from the time difference in positions is equal to or lower than a predetermined value which is higher than zero, and that the change of the direction of the velocity vector is large. This technique is based on the fact that the lower the velocity of a vehicle becomes, the lower the detection accuracy of the velocity vector becomes. According to the technique disclosed in PTL 3, it is possible to reduce the possibility of determining that a vehicle has stopped although the vehicle has not stopped in reality.